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The authors of this paper (N Shaukat, J Lear, A Lowy, S Fletcher, D P de Bono, K L Woods. BMJ 1997;314:550-4) have written: “Further examination of the data on which this paper was based, in the context of another project, has revealed important inaccuracies such that the conclusions of the paper cannot be sustained. We therefore wish to withdraw it unreservedly.”